But the act remains on life support. It has not been reauthorized—given multiyear funding—since the late 1980s, subsisting instead on annual appropriations requested by the Interior Department. The Bush Administration has done nearly everything it could to endanger the act itself by decreasing funding and politicizing the scientific evaluations that determine the status of species at risk. As this article goes to press, only 64 species have been listed in the almost eight years George W. Bush has been in the White House. During his father's four years, the total was 235.

There's nothing easy about adding a creature to the list. Sometimes a species is proposed by the Fish and Wildlife Service or the National Marine Fisheries Service. Sometimes it's proposed by the public or a conser­vation group. A candidate for listing must undergo a scientific review and a public comment period. One of the most recent additions—the polar bear, which was given threatened status last May—suggests some of the inherent difficulties. Polar bear habitat is dwindling due to climate change, but it's also being compromised by the rush to exploit the Arctic for minerals and petroleum. Dirk Kempthorne, the Secretary of the Interior, listed the polar bear only after being forced to by a federal court, and only after calling the Endangered Species Act "perhaps the least flexible law Congress has ever enacted." In its waning months the Bush Administration pro­posed regulatory changes that would gut the act by allowing federal agencies, not scientists, to decide whether to protect a species.

At present 1,050 species in the United States and its neighboring waters are listed as endangered—at risk of extinction. Another 309 are listed as threatened, or likely to become endangered in the foreseeable future. There are recovery plans—strategies for restoring dwindling populations—for most of them, including measures like acquiring critical nesting beaches for Atlantic loggerhead sea turtles or restoring wetland habitat for the copperbelly water snake. But critical habitat has been designated for only 520 species. And when it comes to actual recovery, the numbers are not encouraging. Since 1973, only 39 U.S. species have been removed from the endangered and threatened list. Nine of those went extinct, and 16 were removed when evidence emerged that a listed species was not, in fact, imperiled. Only 14 have recovered enough to be delisted. Meanwhile, listing is pending for nearly 300 official candidates, everything from Las Vegas buckwheat to the Miami blue butterfly.

Critics say those numbers show how ineffective the Endangered Species Act really is. But the numbers may instead show just how much economic and political inertia the act has faced. And there are other ways to measure its success. How many species might have vanished without it? Perhaps the best measure of the act's value is the very contention it causes, the fact that it gives endangered species a day in court and helps us see the unintended consequences of our actions. It reminds us that what look like simple economic decisions—to build a subdivision or auction new drilling leases or plant more corn for ethanol production—have to be considered within the greater economy of nature, where many lives are in the balance.

Some creatures are so iconic that it's easy to see why we take the trouble to save them. The bald eagle was removed from the list in 2007 because its numbers in the lower 48 states have been successfully restored—from fewer than 500 nesting pairs in 1963 to some 10,000 pairs in 2007. One population of the grizzly bear—in Yellowstone National Park—has graduated from the list. So have such impressive species as the peregrine falcon and the American alligator.